Question: Solve for $r$ : $28 + r = -21$
Subtract $28$ from both sides: $ \begin{eqnarray} \\ 28 + r &=& -21 \\ \\ {-28} && {-28} \end{eqnarray} $ $ \begin{eqnarray} \hphantom{28 + r} &\hphantom{=}& \hphantom{-21} \\ r &=& -21 {- 28} \\ \end{eqnarray} $ Simplify. $ r = -49$